The present invention relates to a sealing device for the driver member of a pneumatically operated driving tool for fasteners.
EP-B 0326 639 discloses a driving tool of this type. For performing a working stroke for driving a fastener into a workpiece, the working cylinder above the working piston is supplied with compressed air. The working piston moving downwardly displaces the air from the cylinder space below the piston through openings into an adjacent piston return chamber. When the working piston reaches its lower dead center which is defined by a stop member, the working space above the working piston is connected to atmosphere and the air accumulated in the piston return chamber drives the working piston back into its upper dead center. A control valve connects the upper cylinder chamber to a source of compressed air, or, respectively, to atmosphere. The control valve allows for performance of a single stroke operation for a repetitious operation.
To avoid a loss of air accumulated in the piston return chamber, the driver member must be provided in a sealing engagement with the cylinder. To accomplish this, the driver member may closely fit into a slot provided in the stop member. However, this has the drawback that a certain volume of air may be caught within the space below the working piston which is detrimental to a fast driving operation. Furthermore, the slot may be widened or otherwise damaged when the working piston hits the stop member which would adversely effect the sealing engagement. This might be remedied by providing a metallic disk to be the sealing element. The disk is supported between the stop member and a head piece at the bottom of the cylinder for guiding the drive member in a tight slot. Again, air below the working piston adversely effects a fast operation.
According to another structure of the prior art, the driver opening includes a vent passage or groove for venting the residual air. This vent groove extends from an opening in the stop member towards the forward end of the driver member located in the mouth tool. When the piston is located in its upper dead center, the residual air may be vented through the vent groove opening into the driver channel. In the downward stroke of the working piston, the driver member completely covers the groove after a short travel of the piston to seal the passage between the vent groove and atmosphere so that the accumulation of the return air is initiated.
It should be understood that problems are encountered in sealing the vent passage with respect to the working cylinder. It is known to squeeze an upper portion of the mouth tool into a metallic head piece supporting the resilient stop member. However, the squeezing operation is not easily performed when the plate-shaped sections of the mouth tool need to be accurately placed with respect to each other. Furthermore, the head piece may include a guiding and sealing means for the driver member, the mouth tool sealingly resting upon it. Although a squeezing operation is not required, the blade-shaped mouth tool portions must be accurately levelled to define a plane support surface for the driver member guiding means made of plastic material.